1. Field of the Invention
The present invention relates to an image forming apparatus, and, in particular, to an image forming apparatus for forming an image electrostatically.
In an image forming apparatus using the electrophotographic recording method for performing color printing, toners having a plurality of colors such as yellow, cyan, magenta and black are transferred onto a recording medium so as to be overlaid on each other so that color printing is performed. At this time, the toners are powder and may scatter so as to stain recording paper and/or the apparatus as transfer dust. Therefore, it is necessary to reduce scattering of the transfer dust.
2. Descriptions of the Related Art
FIG. 1 shows a general arrangement of one example of the related art.
A color printer 100 using the electrophotographic recording method includes electrostatic recording units 102-1 through 102-4 for four colors: yellow (Y), magenta (M), cyan (C) and black (K), for electrostatically recording a toner image, a fixing unit 103 for fixing a color image, recorded onto a recording paper 101, recorded by the electrostatic recording units for the four colors, on the recording paper 101, and a carrying mechanism 104 for carrying the recording paper 101.
The recording paper 101 is drawn out from a hopper 105 by the carrying mechanism 104, and is carried to the recording units 102-1 through 102-4 for the four colors. The electrostatic recording units 102-1 through 102-4 for the four colors are disposed in the direction (the direction of the arrow C) in which the recording paper 101 is carried tandem, and transfer the toners of the four colors onto the recording paper 101 so as to be overlaid on each other sequentially.
The recording paper 101 is carried in the direction of the arrow C by the carrying mechanism 104, and, is supplied to the fixing unit 103 after the toners of the four colors are transferred onto the recording paper 101 in the order of yellow (Y), magenta (M), cyan (C) and black (K) by the respective electrostatic recording units 102-1 through 102-4.
The fixing unit 103 fixes the toners, transferred onto the recording paper 101 by the electrostatic recording units 102-1 through 102-4 for the four colors, by means of heating and pressing. The recording paper 101, on which the toners have been fixed by the fixing unit 103, is further carried by the carrying mechanism 104 and is stacked on a stacker 106.
FIG.2 shows a general arrangement of the electrostatic recording unit in one example of the related art.
Each of the electrostatic recording units 102-1 through 102-4 for the four colors includes a photosensitive drum 107 on which an electrostatic latent image corresponding to a recording image is formed, an electric charger 108 for electrically charging the photosensitive drum 107 uniformly, an LED array 109 for irradiating the photosensitive drum 107, which has been electrically charged uniformly, in accordance with the recording image, a developer 110 for developing the electrostatic latent image formed on the photosensitive drum 107 using the toner, and a transfer roller 111 for transferring the toner image developed by the developer 110 on the photosensitive drum 107 into the recording paper 101.
At this time, in the electrostatic recording units 102-1 through 102-4 in the related art, in order to improve the toner transfer efficiency for transferring the developed image onto the recording paper, the polarity of the electric potential of the transfer roller 111 is set to be reverse of the polarity of the electric potential of the toners.
Further, the electric potential of the transfer roller 111 is set to be the same between the electrostatic recording units 102-1 through 102-4.
Thereby, for example, when the toner is transferred so as to be overlaid on the previously transferred toner on the recording paper by the electrostatic recording unit, the tone of the currently transferred toner is lowered due to the influence of the previously transferred toner. Further, because a distance occurs between the photosensitive drum 107 and the recording paper 101, unnecessary toner is transferred onto the recording paper 101, that is, the transfer dust occurs. Thereby, the printing quality is degraded.